In the field of aeroelasticity, the phenomenon known as flutter generally refers to a condition produced by the coalescing and proper phasing of two or more structural vibration modes of a structure, such as an aircraft wing, fuselage, empennage, or other structural component. During flight, the aerodynamic forces over such structures may cause an unstable oscillatory aeroelastic deformation of the structure referred to as flutter. Flutter of an aeroelastic structure may depend on numerous factors, including the mass, stiffness, and shape of the structure, and the particular operating conditions of the structure, including the velocity and density of the airstream.
Flutter of an aircraft wing may involve both bending and torsional types of motion. In some cases, the aeroelastic deformations may be relatively mild and stable within the normal operating envelope of the aircraft. In the case of flutter, however, the aeroelastic deformations are driven into an unstable mode in which the torsional motion extracts energy from the airstream and drives the bending mode to increasingly higher amplitudes, causing oscillations of increasing amplitude that eventually result in catastrophic failure of the structure.
The avoidance of the unstable condition of flutter and the determination of the maximum allowable flight speed before flutter is encountered are critical priorities for designers of aeroelastic structures and aerospace vehicles. Exhaustive flight and wind tunnel tests are usually conducted to record and observe the flutter characteristics of the various aeroelastic structures of an aircraft over the entire operating envelope of the aircraft. For military aircraft, the attachment of external stores to the aircraft further complicates the analysis and increases the extent and complexity of the testing due to the multiple store configurations dictated by modern combat, surveillance, and fuel capacity requirements. These differences in store number, type, and mounting location give rise to complex multi-variable oscillation coupling patterns, and can give an aircraft as many different flutter speeds as there are store configurations.
The acquisition and analysis of flutter test data may be difficult and expensive, particularly when considerable flight testing is involved. The two principal methods of acquiring flutter flight test data are so-called dwell and decay tests, and sine sweep (or frequency response) tests. The transfer function frequency response data are generally used to identify frequency areas of interest to be investigated further with dwell and decay time history methods. The flight time required for the dwell and decay method is extensive compared with the flight time required to obtain the frequency response data. Although desirable results have been achieved using the previously existing test and analysis methods, the complexity of aeroelastic flutter and the continuing advances in aerospace vehicle requirements are placing increasing demands on those who design and analyze flutter test data of aeroelastic structures. Therefore, there is an unmet need for improved methods of analyzing flutter test data, particularly those which may provide improved results or which may reduce the required amount of flight testing.